3) Mongul
Supergirl has featured a number of major Superman enemies over its first two seasons – Metallo, Parasite, Bizarro, Livewire and even a brief cameo of General Zod. It makes sense that the show will continue to raid the Big Blue Boy Scout’s rogues gallery, then, and at the top of the list should be Mongul.
Mongul is a fearsome alien dictator of Warworld, a vast outer-space empire where he entertains himself and his people with gladiatorial games. He’s also a formidable fighter on his own and is one of the few foes who can go toe-to-toe with – and occasionally beat – Superman in a one-on-one fight. Seeing as last season started sending Kara to other planets, it wouldn’t be a big leap for her to visit Warworld in season 3. It even got a namecheck in a recent episode.
Yes, the limits of Supergirl‘s budget would probably mean that the show couldn’t go all out in adapting the cosmic warlord and his world, but the villain is one who’s unfortunately never made it to live-action before and Supergirl has the chance to right that wrong.